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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Wed, 12 Feb 2020 12:59:22 -0800
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Subject: Re: [PATCH v7 11/11] arm64: scs: add shadow stacks for SDEI
To:     James Morse <james.morse@arm.com>
Cc:     Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Kees Cook <keescook@chromium.org>,
        Laura Abbott <labbott@redhat.com>,
        Marc Zyngier <maz@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Jann Horn <jannh@google.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
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On Tue, Feb 11, 2020 at 5:57 AM James Morse <james.morse@arm.com> wrote:
>
> Hi Sami,
>
> On 28/01/2020 18:49, Sami Tolvanen wrote:
> > This change adds per-CPU shadow call stacks for the SDEI handler.
> > Similarly to how the kernel stacks are handled, we add separate shadow
> > stacks for normal and critical events.
>
> Reviewed-by: James Morse <james.morse@arm.com>
> Tested-by: James Morse <james.morse@arm.com>

Thank you for taking the time to test this, James!

> > diff --git a/arch/arm64/kernel/scs.c b/arch/arm64/kernel/scs.c
> > index eaadf5430baa..dddb7c56518b 100644
> > --- a/arch/arm64/kernel/scs.c
> > +++ b/arch/arm64/kernel/scs.c
>
> > +static int scs_alloc_percpu(unsigned long * __percpu *ptr, int cpu)
> > +{
> > +     unsigned long *p;
> > +
> > +     p = __vmalloc_node_range(PAGE_SIZE, SCS_SIZE,
> > +                              VMALLOC_START, VMALLOC_END,
> > +                              GFP_SCS, PAGE_KERNEL,
> > +                              0, cpu_to_node(cpu),
> > +                              __builtin_return_address(0));
>
> (What makes this arch specific? arm64 has its own calls like this for the regular vmap
> stacks because it plays tricks with the alignment. Here the alignment requirement comes
> from the core SCS code... Would another architecture implement these
> scs_alloc_percpu()/scs_free_percpu() differently?)

You are correct, these aren't necessarily specific to arm64. However,
right now, we are not allocating per-CPU shadow stacks anywhere else,
so this was a natural place for the helper functions. Would you prefer
me to move these to kernel/scs.c instead?

Sami